As having high energy density and high capacity, a lithium ion secondary battery is widely used as a power supply for driving a mobile information terminal or the like. In recent days, it is also used for an industrial application like mounting in an electric • hybrid automobile or the like wherein high capacity is needed, and therefore studies to achieve even higher capacity and higher performance are carried out. One of the studies is to increase charge and discharge capacity by using silicon or tin as a negative electrode active material having a great lithium occlusion amount per unit volume, or an alloy containing them.
However, when silicon, tin, or an alloy containing them, which are an active material having high charge and discharge capacity, is used, a great volume change in the active material is caused in accordance with charge and discharge. As such, an electrode using polyfluorovinylidene or rubber-based resin as a binder resin, that have been widely used for an electrode in which carbon is used as an active material, has a problem that the active material layer is easily degraded or peeling occurs at an interface between a current collector and the active material so that the current collecting structure within the electrode is destructed and electron conductivity of the electrode is lowered, and as a result, the cycle property of the battery is easily deteriorated.
For such reasons, a binder resin composition which hardly undergoes any destruction or peeling of an electrode even under a significantly high volume change and has high toughness under battery environment has been waited for.
As disclosed in Patent Literature 1, use of a polyimide resin as a binder for an electrode of a lithium ion secondary battery is well known.
It is suggested in Patent Literatures 2 and 3 to use binder resins each having a certain mechanical property for an active material consisting of a silicon alloy or an alloy containing tin. However, specific chemical structures of the resins are not disclosed.
In Patent Literature 4, a lithium secondary battery including an active material which consists of silicon and silicon-based alloy and a polyimide resin having a specified chemical structure used as a binder is suggested. The polyimide resin is polyimide having a residue of 3,3′,4,4′-benzophenone tetracarboxylic acid.
Meanwhile, it is described in Non Patent Literature 1 that lower swelling degree of a binder resin for an electrode in an electrolyte solution yields higher discharge capacity retention ratio according to charge and discharge cycle, and therefore desirable.
Further, in Non Patent Literature 2, the reductive decomposition of an electrolyte solution within a lithium battery is studied and generation of methoxy lithium and the like on surface of the electrode is shown. Thus, under battery environment, methoxy lithium having a strong alkali property and a potentially negative effect on the binder resin is included in the electrolyte solution.